I think you are confusing the “ninja edit window” and the “post edit time limit” settings.
The former is used to deal with versions of the post while the later is used to prevent users from editing/deleting their post after a certain amount of time.
Right now, that setting only applies to post. Which mean that users can change the title, category and tags of their topics with no limit.
So that is intentional? Seems like an odd decision to let a user constantly change the category of their topic including adding/removing tags. So maybe this isn’t a bug but more of a feature to let the topic use the post edit time limit?